1. Field of the Invention
The invention relates generally to metering pumps and specifically to high-precision bellowstype liquid metering pumps with controlled dispense volume and velocity.
2. Description of the Prior Art
Fluid pumps are available in a variety of configurations, including: diaphragm, bellows, and piston actuated versions. Precision metering pumps favor the diaphragm, bellows, and piston types because of their ability to provide a constant flowrate when the volume being dispensed is less than or equal to the maximum volume the pump is capable of dispensing with a single stroke. In piston actuated pumps the fluid being pumped can coat the cylinder walls and get pass the piston, and so these types are not preferred in high purity applications where contamination of the media must be avoided.
In the piston pump, the volume of fluid being pumped is a direct function of the piston area, times the length of the piston stroke, times the number of strokes. Bellows types are similarly determinable. In fluid dispensing for semiconductor manufacture, the fluid delivery must be in a single evenly pressured stream with no pulsations, therefore, no more than a single full stroke is used. This, of course, limits practical dispense volumes to the piston area times the piston stroke. Since the piston area and stroke length are usually fixed by design, the pump delivery is adjustable by the length of the stroke. Precise control of a piston's stroke length is conventionally accomplished by attaching a threaded lead screw to the piston and by driving a threaded nut on the lead screw with a stepper motor. With diaphragm pumps, there is no way of measuring the stroke by distance travelled, thus other means, such as the time multiplied by the force driving the dispense, must be used as approximations.
In FIG. 1(a), a diaphragm pump, referred to by the general reference character 10, has an air inlet port 12 that puts a gas 14 behind a diaphragm 16. A fluid 18 is input through a check-valve 20 and output through another check-valve 22. The volume of fluid 18 pumped will depend on the pressure behind gas 14 and the duration of the stroke. It is therefore difficult to get precise fluid 18 metering with pump 10. A second problem with pump 10 is that diaphragm 16 has a short life and must frequently be replaced. This shortness of life is mainly attributable to the differential pressures that exist across the membrane of diaphragm 16 during operation and by the high stresses present at the perimeter/seal. An advantage of pump 10 is that it is simple to manufacture.
A hybrid diaphragm/piston pump is shown in FIG. 1(b) and referred to by the general reference character 30. Pump 30 attempts to solve the short diaphragm life problem by placing a non-compressible fluid 32 between a piston 34 and a diaphragm 36. Pump 30 also solves the problem of volume displacement uncertainty by having the piston 34 transfer pumping forces to diaphragm 36. A volume represented by the area of the piston 34 times the stroke length of the piston 34 will be exactly matched by the volumetric displacement of diaphragm 36, and therefore by a fluid 38. Containing the non-compressible fluid 32 behind diaphragm 36 presents a potential for contamination since an absolute perfect seal is virtually impossible to maintain.
A bellows pump is shown in FIG. 1(c) and is referred to by the general reference character 50. A bellows 52 rides on a motor shaft 54 that strokes the bellows 52 within the pump 50 and thereby pumps a fluid 56. Ordinarily, the bellows 52 would be constructed of stainless steel, but in ultra-pure chemical pumping operations, the nickel in stainless steel can leach into fluid 56 and ruin semiconductor wafers in production. The semiconductor industry--which is a major user of precision metering pumps--has, therefore, come to demand that all components that come into contact with the fluids, e.g., 18, 38, and 56, be constructed of a material that does not contaminate the fluid, e.g., Teflon.RTM.. A bellows 52 made of a material that does not contaminate the fluid, for example Teflon, will however, flex under even moderate pressure, unlike a stainless steel bellows. This flexing changes the effective volume of fluid 56 in pump 50 that will be displaced by bellows 52. Backing the bellows 52 with air pressure would be a solution to the problem, but changing downstream conditions that resist the flow of fluid 56 out of pump 50 are hard to compensate for exactly and any mismatch would imbalance the bellows 52 once again.
Bellows-type fluid pumping systems are well known. For example, U.S. Pat. No. 4,483,665 ('665), by Hauser, describes a displacement type fluid and filtering system that uses a longitudinally contracting bellows and a compressed air driven piston to pump fluid through the system. A suck-back mechanism is shown by Hauser ('665) for preventing fluid from remaining at the nozzle tip after a discharge stroke. Hauser ('665) uses a check valve to relieve pressure that builds up during the discharge stroke and a cartridge filter to filter the fluid before it is discharged. In a related patent by Hauser, U.S. Pat. No. 4,541,455 ('455), there is an automatic vent valve for use with a bellows-type fluid pumping and filtering system. The valve automatically vents gases that build up during the cycling of the bellows-type pump.
Prior art bellows-type chemical pumping systems use open loop control systems. The rate of fluid discharge is controlled by the air flow rate that drives the bellows compression piston. Such systems are open loop because there is no feedback information available to the systems for modulating the air pressure in response to an unsatisfactory rate.
A common fluid dispensed in precision volumes by pumps 10, 30, and 50 is photoresist. Since photoresist contains highly volatile solvents and is prone to outgassing after short standing times, a dribble or positive meniscus on the output nozzle of a pump cannot be allowed to form. If such a dribble or bulge did form and was allowed to stand, it would dry out and drop solid particles in the next dispensing cycle, and would probably ruin the semiconductor wafer being processed. Various schemes have been devised to retract the fluid meniscus. A very practical one places a small chamber with a diaphragm and only one port in a "T" connection with the pump's output nozzle. After the dispensing cycle, the small diaphragm is retracted, causing the suck back of a small volume of fluid from the nozzle, thus causing a negative meniscus, at minimum, that will resist drying out.
Although chemical fluids used in precision metering pumps are quite pure, filters are nevertheless used in association with such pumps. Filter membranes, especially ultra-fine membranes, can be damaged or compromised by (1) attempting to pump air through them, and/or (2) by reverse flowing fluid through the filter. Either case will flex the filter membranes and can cause otherwise trapped contaminants to become dislodged. A simple pump system that consists of one of the pumps shown in FIG. 1 and a filter can flex the filter's membrane when the pump is reversed to effectuate a suck-back. The filter's membrane will also be flexed by air being forced through it when the pump is started for the first time or a new filter cartridge has been installed.